The present invention relates to a method for heating metal slabs in an elongated heating furnace. The invention relates also to an elongated heating furnace for heating metal slabs.
The purpose of heating metal slabs is to bring about a proper temperature for metal slabs to be rolled or otherwise moulded. For example, steel slabs are heated prior to a rolling operation to the temperature of appr. 1100.degree.-1200.degree. C. in order to make them sufficiently mouldable. For heating, the slabs are carried into a heating furnace by means of suitable equipment. The heating furnace is an elongated and usually a brickmasoned structure which is heated with suitable burners. The temperature profile of a heating furnace is selected to be such that, upon leaving the heating furnace, the slabs have a desired temperature. In this context, the term temperature profile is defined as referring to the temperatures of the furnace and slabs contained in the furnace at a certain instant of time. The furnace has its own temperature profile. Respectively, the slabs have their own temperature profile at each point in the direction of thickness. Thus, if the temperature profile of a heating furnace is known, the temperature at each point within the furnace is also known. If the temperature profile of a certain point, e.g. that of the surface of a slab, is known, the surface temperature of the slab will be known at each point in the longitudinal direction of the heating furnace.
A typical heating furnace for metal slabs is divided into three zones, the first zone being a so-called convection zone which is the first to take up the slabs. This is followed by a so-called heating zone which accommodates burners operating on a fossil fuel, i.e. gas or oil. The final zone in the traveling direction of the slabs is a so-called equalizing zone, wherein temperature of the slabs is allowed to equalize. The heating furnace operates in a manner that the combustion gases from burners firing in the heating zone are conveyed upstream relative to the traveling direction of the slabs from the heating zone to the convection zone. Thus, the burners are located in the rear section of the heating zone and a chimney in the front section of the convection zone. With an arrangement like this, a temperature difference between the combustion gas and the slab is significant at each point in the lengthwise direction of a furnace so as to provide a heat transfer as effective as possible from combustion gas to slab. In the convection zone, the transfer of heat from combustion gas to slab occurs through convection, as suggested by the name of this zone.
U.S. Pat. No. 4,299,565 discloses a heating furnace wherein metal slabs are heated by fuel combustion flames. Heat transfer converters each made of a heatresistant material are disposed downstream of the flow of the combustion flames in the convection zone of the furnace. These converters are heated through convection heat transfer from a high temperature and high speed flow of the combustion flames and combustion gas. The purpose of the converters is to raise the thermal efficiency of the furnace by utilizing the heat that otherwise would be lost along with the discharging combustion gases.
Patent application GB 2 048 440 discloses a heating furnace for metal slabs, wherein the metal slabs are heated by using a plurality of burners and radiant heaters located one below another and at equal distances over the entire furnace length. The cited heating furnace is primarily intended for use in the bright annealing of copper under a shielding atmosphere. In bright annealing, a copper product is heated in a reducing atmosphere for removing an oxide layer from the product surface and for producing a bright surface. The purpose of the burner is not only the production of heat but also the production of a reducing atmosphere. The fuel comprises e.g. natural gas or other pure hydrocarbon and the combustion is effected with depleted oxygen for the production of reducing gases, such as carbon monoxide and hydrogen. These gases remove the oxide layer of the surface and brighten the surface. This is followed by cooling the product prior to its discharge from the furnace. It should also be noted that the furnace does not include a convection zone and the combustion gases are discharged from both ends of the. furnace. The furnace has such a temperature profile that temperature is at its highest in the middle of the furnace and becomes lower towards each end. The metal products are carried upon rollers. This type of furnace is not suitable e.g. for heating steel slabs intended for subsequent rolling.
The above-described prior art heating furnaces for metal slabs have several functional deficiencies. Especially, the heating furnaces generally used by the steel industry have a temperature profile which is "inflexible" and also highly dependent on the furnace geometry. Therefore, an effort is made to maintain the temperature profile of a heating furnace as constant as possible. It is prior known that the performance of a heating furnace is very difficult to control by the regulation of burners and, thus, the temperature profile of a furnace cannot be changed in a sufficiently flexible fashion. The furnace capacity is determined on the basis of the thickest slab. A flexible regulation of the temperature profile would be a particularly important feature whenever the metal slabs loaded into a heating furnace have varying thicknesses. Another deficiency found in the prior art heating furnaces is that the increase of capacity without major structural modifications is not generally possible. If the capacity of a heating furnace is to be increased, this could be effected e.g. by extending and/or lowering the furnace, increasing the number of burners or by other such measures. Structural changes are generally required and those are very expensive to carry out. In addition, such measures usually deteriorate the furnace efficiency.
An object of the invention is to provide a method and a heating furnace for eliminating the above drawbacks. Thus, an object is to provide a heating method and an improved heating furnace, e.g., in a manner that the temperature profile of a heating furnace can be flexibly regulated. In addition, it is necessary that the method and the improved furnace can be used for raising the capacities of prior art heating furnaces without major structural modifications thereto.
A method of the invention improves the performance of a heating furnace particularly for the reason that a temperature difference between the top and bottom surface as well as between the top and bottom surfaces and the mid-section of metal slabs decreases without having to reduce the capacity. The reason for this is that a properly positioned radiant heater is capable of providing a rapid heat transfer and, thus, temperature of a metal slab rises at an earlier stage. The equalizing time of temperatures will be longer and the temperature differences within a slab will be reduced. This will improve the rollability of a metal slab as well as the internal quality of a product. This also facilitates the production of thinner hot-rolled products, which offers a major economic benefit as some of the expensive cold-rolling can be replaced this way. The uniformity of slab temperature is of major importance e.g. in terms of the smooth dissolution of alloy materials contained in metal, which in turn affect the mechanical and other properties of the product. In addition, the temperature differences of a slab have an effect on metallurgical phenomena occurring in a slab as well as on the stress condition, which has significant effects on the properties obtained for a product in a rolling operation.
A properly positioned radiant heater is also capable of reducing the lengthwise temperature differences appearing in a metal slab. This facilitates particularly the rolling of thin webs.
A method of the invention can be used for lowering the heating temperature of metal slabs without having to reduce capacity. Lowering of the heating temperature improves the mechanical properties of certain rolled products while affording the possibility of reducing the amount of alloy materials. This leads to an improved weldability and a more economical product.
With a traditional heating furnace, said temperature drop means automatically a reduction of capacity. A method and an apparatus of the invention can be used for decreasing or increasing the furnace temperature in a flexible manner without changing the regulating parameters of a furnace.
With a method of the invention it is also possible to increase the capacity of the heating furnace by 25% while decreasing the total energy consumption by 10% and the consumption of fossil fuels by 35%.
A method of the invention also facilitates a slab-by-slab regulation of the slab temperature profile. It is possible to use thicker slabs in the heating furnace without compromising the heating capacity and thus to increase the coil weight. This has a major economic significance in terms of the further processing in a steel works.